Plasmodium life cycle
proteins known in other organisms. Of particular in- terest are highly variable gene families that are clus- tered near the telomeres. Some of these genes en- code proteins that are exported to the surface of the infected red blood cell where they bind to receptors in various tissues of the host. P. falciparum appears to have arisen within the past 6,000 years as the re- sult of the lateral transfer between bird and human hosts. See Appendix C, 2002, Gardner et al.; Appen- dix E; Appendix F; apicoplast, artemisinin, Plasmo- dium, Plasmodium life cycle, pfcrt gene. Plasmodium life cycle the developmental changes which the malaria parasite, Plasmodium (q.v.) un- dergoes as it is transmitted from a vector to a host, and back to a vector. The life cycle of P. falciparum, the major parasite causing human malaria, is shown in the illustration. The saliva of an infected Anophe- les (q.v.) female transmits the parasite in the sporo- zoite form when it bites a human being. Each sporo- zoite synthesizes a circumsporozoite (CS) protein (q.v.), which serves as an immune decoy. The sporo- zoites reach the liver, where they invade hepato- cytes. They then undergo schizogony, a replicative process involving multiple rounds of rapid mitoses without cytokinesis (q.v.).
Subsequently, the cyto- plasm segments within the multinucleate mass to form hundreds of uninucleate merozoites. Following rupture of the hepatocyte, the merozoites enter the circulatory system and invade red blood cells. Within the erythrocyte the parasite transforms into the trophozoic or feeding stage, ingesting mas- sive mounts of hemoglobin (q.v.) from the cyto- plasm of its host. The trophozoite hydrolyzes the globin into small peptides and releases the heme, which is converted in the food vacuole (q.v.) to a non-toxic crystalline form, called hemozoin. Shortly after its infection the erythrocyte forms thousands of projections on its surface, which contain adhesion molecules produced by the parasite. The infected cells can now adhere to the walls of the blood vessels and avoid being swept into and destroyed by the spleen. The trophocyte stage ends when the cells un- dergo schizogony.
The host erythrocyte then rup- tures, releasing merozoites which invade new eryth- rocytes. Erythrocytic schizogony is synchronized, causing cells to rupture in synchrony. This is the rea- son for the cyclic fever paroxysms experienced by humans suffering from malaria. The merozoites in some of the erythrocytes differ- entiate into male and female gametocytes, and the erythrocytes containing these do not rupture. These gametocytes can be taken up by mosquitoes in the next blood meal, and it is in the mosquito that the sexual phase of the parasite life cycle occurs. Here male and female gametes fuse, resulting in the only diploid stage, the ookinete. This motile cell trans- verses the midgut epithelium of the mosquito and comes to rest between the epithelial cells and the basal lamina (an acellular membrane surrounding
the outside of the gut). The ookinete now completes meiosis to form haploid nuclei, which divide within a sporocyst to form thousands of sporozoites. When the mature sporocyst ruptures, the motile sporozo- ites penetrate the basal lamina and enter the hemo- coel. The sporozoites now migrate to the salivary glands, traverse the epithelial cells of the gland, and come to rest in the lumen. The life cycle is com- pleted when the mosquito transfers sporozoites to a human during a blood meal. See Appendix C, 1880, Laveran; 1898, Ross; 1899, Grassi; 1948, Shortt and Garnham; hemoglobin C, sickle cell trait, vacuoles. Plasmodium yoelii yoelii a species that causes malaria in wild African rats which can be readily bred in the laboratory. P. yoelii is used for compara- tive genomic and proteomic studies with P. falci- parum. The yoelii genome has been sequenced and found to contain 23.1 mbp of DNA. There are about 5,800 genes spread among 14 chromosomes.
The A+T content is 77%. The telomeric regions of chromosomes contain large numbers of genes that are homologs of those that function in P. vivax to evade the host’s immune defenses. plasmogamy the fusion of protoplasts of two hap- loid cells without the fusion of their nuclei, as in cer- tain fungi. plasmon all extrachromosomal hereditary agents considered collectively. plasmosome a term in the older literature refer- ring to the nucleolus (q.v.).
plasmotomy fission, unrelated to nuclear division, of a multinucleated protist into two or more multi- nucleated sibling cells. plastid a self-replicating cytoplasmic organelle of algal and plant cells, such as a chloroplast, chromo- plast, elaioplast, or leukoplast. plastome a plastid genome. plastome-genome incompatibility a form of ge- netic disfunction that affects plastid development. Examples are plastids inherited from one parent which fail to become fully pigmented in the nuclear background of species hybrids. plastoquinone a group of quinones (q.v.) involved in the transport of electrons during photosynthesis in chloroplasts. plate 1. a flat, round dish (petri plate) containing agar and nutrients for the culture of bacteria. 2. to spread or inoculate cells on the surface of semisolid medium in such a culture dish. 3. a geological plate. See plate tectonics. platelet-derived growth factor (PDGF) a protein synthesized by platelets that is released into the se- rum during blood clotting. PDGF represents the ma- jor growth factor in human serum, and it is a potent mitogen for connective tissue and glial cells. There are extensive similarities between the amino acid se- quences of PDGF and the product of the v-sis onco- gene of the simian sarcoma virus (q.v.).
This suggests that v-sis resulted from viral recombination with a host gene encoding PDGF. See Appendix C, 1983, Doolittle et al.; proto-oncogenes. platelets anucleate, oval, colorless corpuscles pres- ent in blood. Platelets, which are one-third to one- half the size of erythrocytes, originate from projec- tions pinched off the surface of megakaryocytes (q.v.), and function in blood clotting. See thrombo- cytopenia. plate tectonics a theory that provides an explana- tion for the present-day global distribution of moun- tain building, volcanism, and earthquake activity along a series of linear belts. The theory postulates that the surface of the earth is a rigid outer shell, the lithosphere, which lies on a hotter semiplastic athenosphere. The brittle lithosphere is broken into a series of tectonic plates that move horizontally across the earth’s surface. The plates are at once jos- tling and being constrained by neighboring plates, like an ever-shifting mosaic of tiles that change shape as they fill in intervening gaps. It is at plate boundaries that mountains are built and volcano and earthquake activities occur. Present-day continents are parts of tectonic plates, and they ride passively as magma comes to the surface and then moves lat- erally as the sea floor spreads.
The driving mecha- nism of the convection currents is heat from the de- cay of radioactive elements in the earth’s core. Over long periods of time, rocks in the mantle deform like a fluid and move at rates in the order of centimeters a year. The movement of tectonic plates has been going on for at least 600 million years and some- times can provide explanations for the biogeo- graphic distribution of plants and animals. See Ap- pendix C, 1968, Morgan, McKenzie, and Le Pinchon; biogeographic realms, continental drift, sea floor spreading, Sulawesi. plating efficiency See absolute plating efficiency, relative plating efficiency. platyrrhine referring to primates of the infraorder Platyrrhini that includes the New World monkeys, marmosets, and tamarins. These primates are char-
acterized by nostrils that are far apart and face to the sides, and they have prehensile tails. Compare with catarrhine. playback experiment an experiment designed to recover a DNA strand that has been saturated with RNA (see RNA-driven hybridization), and then using it in a further reassociation reaction to show that its C0t1/2 (q.v.) corresponds to that expected of nonre- petitive DNA. plectonemic spiral a spiral in which two parallel threads coil in the same direction about one another and cannot be separated unless uncoiled. The com- ponent strands of a DNA duplex are plectonemically coiled. See paranemic spiral, relational coiling. pleiomorphism the occurrence of variable pheno- types in a genetically uniform group of organisms. See phenotypic plasticity. pleiotropy the phenomenon in which a single gene is responsible for a number of distinct and seemingly unrelated phenotypic effects. Pleistocene the ice age, lasting from 10,000 B.C.
to the beginning of the Pliocene. One of the two epochs of the Quaternary period. Homo erectus ap- peared, then Homo sapiens. See geologic time divi- sions. pleomorphic having more than one form or shape. plesiomorphic 1. in classification, referring to a character state that occurs in the group of organisms being considered, but also outside the group. Traits of this type cannot be used to define the group or to indicate that its members were derived from a com- mon ancestor. 2. in evolution, an original primitive feature thought to have arisen in an ancestor of all the taxa being considered. See apomorphic, clado- gram. Pleurodeles salamanders of two species belonging to this genus, P. waltlii and P. poireti, have been studied both genetically and cytologically.
Working maps of the oocyte lampbrush chromosomes of both species are available. pleuropneumonia-like organisms a group of bac- teria that do not form cell walls. PPLOs are included in the phylum Aphragmabacteria (see Appendix A). One PPLO, Mycoplasma pneumoniae, is the cause in humans of an atypical pneumonia. Pliocene the most recent epoch in the Tertiary pe- riod, during which the first hominids appeared. North and South America joined together, and ad- vanced placental mammals migrated into South America where they killed off many marsupials and primitive placental mammals. See Australopithecine, geologic time divisions. -ploid a combining form used in cytology and ge- netics to designate a particular multiple of the chro- mosome set of the nucleus of an organism, as 16- ploid, 32-ploid, etc. ploidy See polyploidy. plumage pigmentation genes a group of genes controlling pigmentation of chicken feathers. Pig- ment will not be produced unless the gene C is pres- ent. A second gene I, which inhibits pigment forma- tion, is located on a different chromosome.
The White Leghorn breed of chickens has the genotype IICC, whereas the White Plymouth Rock is iicc. See poultry breeds. pluripotent pertaining to any cell or early embry- onic tissue that has a number of possible develop- mental fates but not all of the fates possessed by the zygote; also known as multipotential. See stem cells, totipotent. plus and minus techniques See DNA sequencing techniques. plus ( ) and minus (−) viral strands 1. in a single-stranded RNA virus, a plus strand is one hav- ing the same polarity as viral mRNA and containing codon sequences that can be translated into viral protein. A minus strand is a noncoding strand that must be copied by an RNA-dependent RNA poly- merase to produce a translatable mRNA. 2. in a sin- gle-stranded DNA virus, a plus strand is one con- tained in the virus particle or any strand having the same base sequence. A minus strand has a base se- quence complementary to the plus strand; mRNA can be transcribed from the minus strand. See Balti- more classification of viruses, hairpin ribozyme.
P-M hybrid dysgenesis See hybrid dysgenesis, M strain, P strain. pneumococci bacteria that cause an inflammation of the lungs (classical lobar pneumonia). These bac- teria belong to virulent strains of Streptococcus pneu- moniae that can synthesize an external capsule which contains a complex polysaccharide. See Strep- tococcus. Pneumococcus Transforming Principle substance isolated from heat-killed, virulent pneu- mococci which when added to cultures of living, non-virulent bacteria cause virulent pneumococci to appear. Thus bacteria of one phenotype are trans- formed to an alternative phenotype by a non-living material. The demonstration that the PTP is DNA rather than protein was one of the earliest lines of evidence that genes are made of DNA. See Appendix C, 1928, Griffith; 1944, Avery, MacLeod, and Mc- Carty; transformation. pod corn Zea mays tunicata, a primitive variety of corn characterized by kernels, each of which is en- closed in a husk. podophyllotoxin an alkaloid isolated from the root of the mandrake, Podophyllum peltatum. Podo- phyllotoxin binds to tubulin (q.v.) and prevents it from polymerizing into microtubules. See colchicine, paclitaxel, spindle poison, vinca alkaloids.
Podospora anserina a filamentous fungus living on dung. Its genome size is 34 mbp distributed among 7 chromosomes. The fungus serves as a model for studies of the role of mitochondria in aging. Podospora mycelia undergo a time-dependent change during their culture. The pigmentation of the hyphae increases, they stop growing, and the apical cells die. During this “aging” process certain parts of the mitochondrial DNA become unstable. A mobile intron detaches from a specific gene, circu- larizes, and undergoes amplification. The gene in- volved normally functions to transcribe a subunit of cytochrome c oxidase.
Therefore the energy produc- tion of the cell is compromized, and it dies. See Ap- pendix A, Fungi, Ascomycota; cytochromes, gene amplification, petites. point mutation 1. in classical genetics, any muta- tion that is not associated with a cytologically de- tectable chromosomal aberration or one that has no effect on crossing over (and therefore is not an inver- sion) and complements nearby lethals (and therefore is not a deficiency). 2. in molecular genetics, a muta- tion caused by the substitution of one nucleotide for another. See rotational base substitution. Poisson distribution a function that assigns prob- abilities to the sequence of outcomes of observing no events of a specified type, one event, two events, and so on without limit. Events following a Poisson distribution are completely randomized.
Suppose that within a defined spatial or temporal region one looks at the events that occur in nonoverlapping subregions. A Poisson distribution for the number of events observed in the total region implies that the events occurring in the subregions do not affect each other. A Poisson distribution will not be found if the events are correlated positively (in the case of clumping) or negatively (in the case of mutual re- pulsion). The Poisson is specified by the average number of events per observation, and its mean and variance are equal. The formula of the function is Pi = (mie−m)/i! where m is the mean number of events; i! is the factorial i(i−1)(i−2) . . . (2)(1); e is the base of natural logarithms, and i is the number for which the probability Pi is given. If m = 1.2, for example, the distribution is as shown in the table that appears above. Many natural distributions follow a Poisson, including the number of radioactive disintegrations of a radioisotope in a fixed period of time, or the number of larvae of a particular invertebrate species captured by towing a plankton net through a speci- fied volume of seawater.
The distribution was for- mulated by Simeon D. Poisson (1781-1840), a French mathematician and physicist. pokeweed mitogen a lectin (q.v.) extracted from rhizomes of the pokeweed, Phytolocca americana, that stimulates the proliferation of lymphocytes, particularly mouse T and B lymphocytes. poky the most famous of the mitochondrial mu- tants of Neurospora. These slow-growing, respira- tion-defective fungi have reduced numbers of ribo- somes. This, in turn, is due to deletions in mtDNA that impair the synthesis of rRNA. See petites. pol I, II, III See DNA polymerase. polar referring to water-soluble chemical groups such as a hydrophilic side chain of an amino acid.